Fuel cell power plant including a variable resistive device

ABSTRACT

A fuel cell power plant ( 20 ) includes a variable resistive device ( 30 ). In one example, the variable resistive device ( 30 ) is operationally associated directly with a cell stack assembly ( 22 ). The controller ( 32 ) selectively varies an electrical resistance of the variable resistive device ( 30 ) responsive to an operating condition of the power plant ( 20 ). By using a variable resistive device, a variety of control functions are possible to address various operating conditions of the power plant ( 20 ) or the cell stack assembly ( 22 ).

TECHNICAL FIELD

This disclosure generally relates to fuel cell power plants and moreparticularly to controlling an operating condition of a fuel cell powerplant.

DESCRIPTION OF THE RELATED ART

Fuel cell power plants are well known. Cell stack assemblies and otherknown components operate in a known manner to provide electrical power.The applications for fuel cell power plants vary. Depending on theinstallation, different features and functions are required of differentfuel cell power plants.

It has been proposed to include a voltage limiting device in a fuel cellpower plant assembly for managing an operating condition of theassembly. One approach includes using different devices for differentoperating condition controls. For example, one voltage limiting devicemay be used during a start up operation while a different voltagelimiting device may be used during a shutdown operation. While thatapproach has proven useful, there are limitations.

For example, adding additional devices to a fuel cell power plantintroduces additional cost. It is therefore not possible to add suchdevices in an unlimited manner. Additionally, such voltage limitingdevices tend to be designed for one particular type of fuel cell powerplant and for only one operating condition. Further, such voltagelimiting devices do not address the needs of all conditions within anoperating scenario for which the device is intended. For example, afixed voltage limiting device during a start up operation does notprovide the ability to avoid non-recoverable decay as some of the cellsgo negative.

U.S. Pat. No. 6,887,599 shows one approach to adding an auxiliary loadto control voltage levels during start up and shut down procedures. U.S.Pat. No. 7,041,405 shows an approach for cyclically switching anauxiliary load into and out of a fuel cell stack external circuit.

Even with such improvements, there is a desire in the industry to beable to provide more customized control over various operatingconditions in a fuel cell power plant.

SUMMARY

An exemplary method of controlling operation of a fuel cell power plantusing a variable resistive device includes selectively varying anelectrical resistance of the variable resistive device responsive to anoperating condition of the fuel cell power plant.

In one example, the electrical resistance is selectively variedresponsive to a condition of a cell stack assembly within the fuel cellpower plant.

For different operating conditions, a single variable resistive devicecan be controlled to introduce a different resistance depending on theoperating condition. Using such a device and a control strategyconsistent with the examples disclosed in this description provides theability to customize the control of various operating conditions of afuel cell power plant while minimizing additional cost because there isno need for multiple devices to achieve the multiple functions.

An exemplary fuel cell power plant includes a cell stack assembly. Atleast one other component is operationally associated with the cellstack assembly. A variable resistive device is operationally associatedwith at least one of the cell stack assembly or the other component. Acontroller selectively controls an electrical resistance of the variableresistive device responsive to an operating condition of the fuel cellpower plant.

Various features and advantages will become apparent to those skilled inthe art from the following detailed description. The drawings thataccompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows selected portions of an example fuel cellpower plant.

FIG. 2 is a flow chart diagram summarizing one example control approach.

FIG. 3 schematically shows selected portions of another exampleembodiment.

FIG. 4 is a timing diagram showing one example control signal.

FIG. 5 is another timing diagram showing another example control signal.

FIG. 6 is a timing diagram showing another example control signal.

DETAILED DESCRIPTION

The disclosed examples relate to customized control of various operatingconditions or functions in a fuel cell power plant. In a disclosedexample, a single variable resistive device is used to provide a varietyof control functions. By selecting the resistance based upon theoperating condition, the disclosed examples allow for realizing avariety of control functions for various fuel cell power plant operatingconditions in an economical manner.

FIG. 1 schematically shows selected portions of an example fuel cellpower plant 20, including a cell stack assembly (CSA) 22. The examplepower plant 20 includes at least one other component 24 operationallyassociated with the cell stack assembly 22. The types of components usedin fuel cell power plants are known. Examples include pumps, heatexchangers, accumulators, demineralizers, enthalpy recovery devices,coolant loops and fuel processors. The component schematically shown at24 represents one or all of the other components in the example powerplant 20. Those skilled in the art who have the benefit of thisdescription will realize what types of components are included in thevarious types of fuel cell power plants.

The example of FIG. 1 includes a variable resistive device 30. In thisexample, the variable resistive device 30 is operationally associatedwith the CSA 22. A controller 32 selectively controls the electricalresistance of the variable resistive device 30 responsive to anoperating condition of the fuel cell power plant 20. In some examples,the operating condition will be a condition of one or more portions ofthe fuel cell power plant 20. In some examples, the operating conditionwill depend only on a feature or condition of the CSA 22. The controller32 in one example is programmed to monitor a plurality of differentoperating conditions and to use appropriate electrical resistancesavailable from the variable resistive device 30 to achieve a desiredcharacteristic of an existing operating condition or to provide adesired function, for example.

FIG. 2 includes a flowchart diagram 40 summarizing one example approachthat an example controller 32 utilizes for selecting an appropriateelectrical resistance of the variable resistive device 30 to achieve adesired goal. In this example, the flowchart 40 includes a decision at42 where the controller 32 determines whether the power plant 20 is in astart-up operating condition. Using a voltage limiting device during astart up condition provides advantages and efficiencies. The controller32, therefore, determines if the power plant 20 is in a start-upoperating condition at 42. At 44, the controller 32 selects anappropriate resistance based upon the determination whether the start-upoperating condition exists.

In the illustrated example, the controller 32 has the ability to controlthe electrical resistance of the variable resistive device in aplurality of different manners. As schematically shown at 46, theelectrical resistance may be selected and maintained at a steady valuethroughout the current operating condition. Alternatively, asschematically shown at 48, the controller 32 dynamically varies theelectrical resistance within a particular operating condition. In suchan example, not only does the controller vary the resistance todifferent electrical resistance values for different operatingconditions, but also has the ability to vary the electrical resistancevalue within a particular operating condition.

For example, during a start-up condition the electrical resistance ofthe variable resistive device 30 in one example is dynamically varied tomaintain a constant, low voltage during start-up fuel introduction. Inone example this is accomplished by monitoring the voltage on all thecells of the CSA 22 and responsively varying the electrical resistanceof the variable resistive device 30 to ensure that the voltage on all ofthe cells remains positive. This approach facilitates reducing anynon-recoverable decay that is otherwise associated with a start-upoperating condition.

The ability to dynamically vary the resistance during an operatingcondition may be based upon dynamically determining characteristics ofthe cell stack assembly 22, for example. One example includes a sensorarrangement to provide the appropriate information to the controller 32.In one example, empirical testing is done to determine particularvoltage profiles and associated decay characteristics. The controller 32is provided with a database or information such as a look up table thatincludes corresponding resistance values that should be selected by thecontroller 32 during appropriate portions of a start-up operation toachieve a desired decay characteristic, for example.

The example of FIG. 2 also includes a determination whether water leveldetection is desired at 50. With a variable resistive device 30 asschematically shown in FIG. 1, when the device is appropriately situatedwithin the fuel cell power plant 20, it is possible to use a knowntechnique for making a water level determination using the variableresistive device 30. This example approach has the advantage of making awater level determination even when dedicated water level sensors havenot yet been activated because of the current condition of the sensorsor the power plant 20. When water level detection using the variableresistive device 30 is desired, the controller 32 selects an appropriateresistance at 44.

Another feature available from the illustrated example is to provide athawing function, which may be needed for some freeze capable fuel cellpower plant installations, for example. At 52, the controller 32determines whether thawing is needed. By having the selectively variableresistive device 30 appropriately situated within the power plant 20, itis possible to use that device as a heater, for example, for providing athawing function. When thawing is needed, the controller 32 selects anappropriate resistance at 44.

Another function available from the illustrated example is a freezeprotection function. The controller 32 makes a determination at 54whether freeze protection is desired during operation or subsequent tooperation of a fuel cell power plant before freezing may have occurred.When freeze protection is desired, an appropriate resistance for thevariable resistive device 30 is selected and utilized.

A voltage trim function is available at 56. There are various operatingconditions where trimming a voltage of one or more cells in the CSA 22,for example, may be desired. The controller 32 in one example isprogrammed to determine when such a condition exists and to control thevariable resistive device 30 in a corresponding manner to achieve thedesired effect.

At 58, the controller 32 is able to determine whether a power plant turndown operating condition exists or is desired. If so, the controller 32makes an appropriate resistance selection at 44 to control the variableresistive device 30 to achieve the desired effect.

A voltage limiting device can be useful during a shutdown procedure of afuel cell power plant. The example of FIG. 2 includes a determination at60 whether a shutdown procedure is ongoing or about to be implemented,for example. If a voltage limiting function within a shutdown procedureis desired, the controller 32 selects an appropriate resistance toachieve the desired effect. In one example, the resistance used forshutdown is different than that used for power plant start-up, forexample.

Another function available in the example of FIG. 2 is a shorting strapfunction. At 62, the controller 32 determines whether a shorting strapfunction is desired and appropriately controls the variable resistivedevice 30 to provide that function.

It may be possible for each of the resistance determinations in theexample of FIG. 2 to be different electrical resistances. In someexamples, some of the electrical resistances for different operatingconditions will be the same. Given this description, those skilled inthe art will be able to select appropriate resistance values forcorresponding operating conditions of the particular fuel power plantwith which they are dealing.

As can be appreciated, a single variable resistive device 30 and anappropriate control strategy allows for providing a variety of functionsto achieve various desired characteristics of different operatingconditions for a fuel cell power plant. The illustrated example,therefore, provides the advantage of minimizing expense by minimizingthe number of components required to provide a variety of advantageouscontrol functions within a fuel cell power plant assembly.

In one example, the variable resistive device 30 is operationallyassociated directly with the CSA 22 as schematically shown in FIG. 1. Inanother example schematically shown in FIG. 3, a variable resistivedevice 30 is operationally associated directly with at least one othercomponent 24 of a fuel cell power plant 20. Given this description,those skilled in the art will be able to select an appropriate way ofincorporating a variable resistive device into an appropriate portion ofa fuel cell power plant to meet their particular needs.

In one example, the controller 32 uses a control signal to selectivelyvary the electrical resistance of the variable resistive device. In anillustrated example as schematically shown in FIG. 4, a control signal70 comprises a plurality of pulses 72, 74, 76, etc. In this example, thecontroller 32 uses pulse width modulation on the control signal 70 toselectively vary the electrical resistance provided by the variableresistive device 30. In one example, selectively varying the duty cycleof the control signal achieves the various electrical resistances neededfor the various operating conditions. FIG. 5 schematically shows acontrol signal 70′ where pulses 72′-76′ have a shorter on time comparedto those in FIG. 4. As can be appreciated from FIGS. 4 and 5, adifferent duty cycle is used in each instance. In one example, thecontrol signal 70 as schematically shown in FIG. 4 is used to achieve afirst electrical resistance for a first operating condition of the fuelcell power plant 20. The control signal 70′ is used to achieve a second,different electrical resistance for a second, different operatingcondition.

In an example where the controller can dynamically change the electricalresistance even during a particular operating condition or responsive toa particular characteristic or condition of the CSA 22, a controlstrategy as schematically shown in FIG. 5 as used in one example. Acontrol signal 80 in this example includes pulses 82 and 84 of a firstduration. When a corresponding change occurs in a voltage of the cellstack assembly 22, for example, a different resistance is desired inthis example. The controller 32 responds by altering the duty cycle ofthe control signal 80 to provide longer pulses at 86, 88 and 90, forexample.

In one example, the variable resistive device 30 comprises a resistorand a plurality of switches such as MOSFETs that are arranged to respondto a control signal from the controller 32 such that operating thedifferent switches based upon the selected pulse width modulationachieves the desired resistance provided by the variable resistivedevice 30. Given this description, those skilled in the art will be ableto select an appropriate variable resistive device and an appropriatecontrol arrangement to meet their particular needs.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art. The scope of legal protection canonly be determined by studying the following claims.

1-19. (canceled)
 20. A fuel cell power plant, comprising a cell stackassembly; at least one other power plant component operationallyassociated with the cell stack assembly; a variable resistive deviceoperationally associated with at least one of the cell stack assembly orthe at least one other power plant component, the variable resistivedevice having a selectively variable electrical resistance; and acontroller that controls the electrical resistance of the variableresistive device responsive to an operating condition of the fuel cellpower plant, the controller selecting an electrical resistance of thevariable resistive device to provide at least one of a freeze preventionfunction, a thawing function or a water level detection function. 21.The fuel cell power plant of claim 20, wherein the controller controlsthe electrical resistance to control a load on the cell stack assembly.22. The fuel cell power plant of claim 20, wherein the controllerselects a first electrical resistance of the variable resistive deviceduring a start up of the fuel cell power plant; and selects a secondelectrical resistance of the variable resistive device during a shutdown of the fuel cell power plant.
 23. The fuel cell power plant ofclaim 22, wherein the controller selectively varies the first electricalresistance during the start up of the fuel cell power plant.
 24. Thefuel cell power plant of claim 22, wherein the controller selects atleast one third electrical resistance to thereby provide at least oneother function useful during another operating condition of the fuelcell power plant and the at least one other function comprises at leastone of a shorting strap function; a voltage trimming function; or apower plant turn-down function.
 25. The fuel cell power plant of claim20, wherein the controller uses a resistance selecting signal comprisingpulse width modulation to control the variable electrical resistance.26. The fuel cell power plant of claim 25, wherein the controller variesa duty cycle of the resistance selecting signal to thereby control thevariable electrical resistance.
 27. The fuel cell power plant of claim20, wherein the controller controls the variable electrical resistanceresponsive to a condition of the cell stack assembly.
 28. The fuel cellpower plant of claim 27, wherein the controller controls the variableelectrical resistance to maintain a desired voltage of the cell stackassembly.
 29. The fuel cell power plant of claim 20, wherein thevariable resistive device is electrically coupled with the at least oneother power plant component.
 30. The fuel cell power plant of claim 20,wherein the variable resistive device is electrically coupled with thecell stack assembly.
 31. A method of controlling an operating conditionof a fuel cell power plant having a variable resistive device, themethod comprising selectively varying an electrical resistance of thevariable resistive device responsive to an operating condition of thefuel cell power plant to perform at least one of a freeze preventionfunction, a thawing function, or a water level detection function. 32.The method of claim 31, comprising selectively varying the electricalresistance to control a load on the cell stack assembly.
 33. The methodof claim 31, comprising selecting a first electrical resistance during astart up of the fuel cell power plant; and selecting a second electricalresistance during a shut down of the fuel cell power plant.
 34. Themethod of claim 33, comprising selectively varying the first electricalresistance during the start up of the fuel cell power plant.
 35. Themethod of claim 33, comprising selecting at least one third electricalresistance; providing at least one other function useful during anotheroperating condition of the fuel cell power plant using the at least onethird electrical resistance.
 36. The method of claim 35, comprisingproviding at least one of a shorting strap function; a voltage trimmingfunction; or a power plant turn-down function as the at least one otherfunction.
 37. The method of claim 31, comprising controlling theelectrical resistance responsive to a condition of the cell stackassembly.
 38. The method of claim 37, comprising varying the electricalresistance to maintain a desired voltage of the cell stack assembly.